I would recommend the Galaxy S over the X10. Most carriers don't have the lag fixes available currently, thus the lag. Using a smartphone requires certain knowledge about software, in most cases, android users always 'root' their phone and get the latest dev tweaks (made by normal people, not the phone manufacturers). These updates are ridiculously good, my mate's Galaxy S runs circles around my X10.
The X10 is notorious for having outdated software and for draining battery too fast. Devs haven't been able to fix it because Sony Ericsson spent too much on securing the phone and not enough on making the phone good. Hardware-wise, Galaxy S is the best phone around. Software-wise, the Google Nexus One and the HTC Desire are currently the best ones, with Galaxy S catching up fast.